The present invention relates to an optical memory apparatus employing an optical disc in which information is recorded on the lands between guide-grooves and more particularly to an optical memory apparatus which is capable of optimizing a track cross signal (information reading signal used for cross track counting purposes) and a tracking signal of the diffracted light push-pull type and which is well adapted to an access method of a cross track counting system.
The known optical disc access methods include a cross track counting method in which no external scale is used but tracks formed on an optical disc itself are used as an internal scale so that the number of the tracks traversed by a spot of light during an access operation is counted directly to position an optical head (the light spot) on the desired track. According to this method, as disclosed in U.S. Pat. No. 4,607,358, during the passage (crossing) of the guide grooves (guide tracks), a tracking signal and an information reading signal (track cross signal) including the guide-groove level are both utilized to determine the direction of passage of the light spot and count the number of the guide grooves passed, thereby performing the access operation.
The method of this U.S. Pat. No. 4,607,358 is directed to the optical discs of the groove recording type in which information is recorded along the guide grooves of an optical disc and therefore the guide-groove level must be preset to about 80 to 90% in order to ensure the desired information signal level and to ensure a tracking signal. In this case, the depth of the guide-grooves of the V-shape type is on the order of .lambda./8 (.lambda. is the working wavelength). However, where the cross track counting is effected by using an optical disc having such guide-grooves, there is a disadvantage that the degree of modulation of the guide-groove level signal is as small as 10 to 20% and the guide-groove level signal cannot be detected with a satisfactory S/N ratio thus failing to satisfactorily obtain a polarity signal indicative of the direction of passage of the track.
On the other hand, U.S. Ser. No. 685,123 proposes a method in which information is recorded on the lands between guide-grooves. However, the depth of the guide-grooves is on the order of .lambda./8 and the modulation degree of the guide-groove level signal is as small as in the case of the previously mentioned U.S. Patent. Thus, in order to effect the cross track counting, there is a difficulty from the safety point of view and it is difficult to use the cross track counting.